1. Field of the Invention
The present invention relates to an image capturing apparatus including an electronic shutter and a mechanical shutter.
2. Description of the Related Art
In recent years, there have been proposed image capturing apparatuses for outputting an image obtained by controlling an exposure time by an electronic shutter and an image obtained by controlling an exposure time by a mechanical shutter. Japanese Patent Laid-Open No. 2008-22198 discloses a technique of calculating a shift between the timing of the reset operation of an image sensor and the traveling characteristic of the blades of a mechanical shutter when capturing still images by an electronic shutter and the mechanical shutter.
In a digital camera, there are broadly two kinds of methods of controlling the exposure time of an image capturing apparatus. One method is a mechanical shutter for physically blocking light entering from an object to pixels. The other method is an electronic shutter for electrically controlling resetting of pixels and readout of charges.
A general method of driving a mechanical shutter will be described. The mechanical shutter includes two blade units each pivoted on a shutter plate. Each blade unit rotatably holds a group of a plurality of divided shutter blades by two arms. One blade unit functions as leading blades that cause a shutter opening to transit from a shielding state to a full-open state at the time of exposure. The other blade unit functions as trailing blades that cause the shutter opening to transit from the full-open state to the shielding state at the time of exposure. Each of the blade units respectively forming the leading blades and trailing blades is connected to a driving member including a driving source such as a spring for providing power in a predetermined direction and an armature chucked and held by a control magnet. As the operation of the mechanical shutter, a charge lever moves the mechanical shutter to a position where the blade units are held on a yoke chucking surface. This operation causes the leading blades to shield the shutter opening. When a shutter button is operated, application of power to a coil for the control magnet of the leading blades is canceled, and the leading blades travel to release the shutter opening. After a time corresponding to a shutter speed elapses, application of power to the coil for the control magnet of the trailing blades is stopped, and the trailing blades shield the shutter opening.
On the other hand, as the operation of the electronic shutter, exposure starts after a reset operation of discarding signal charges accumulated in the pixels outside photodiodes. At the time of start of exposure, the photodiodes start to accumulate charges. After a predetermined time, an operation of reading out the charges from the photodiodes starts. The electronic shutter controls the shutter speed by the time from when the pixels start exposure after the reset operation until the charges accumulated in the photodiodes are read out.
In the image capturing apparatus described in Japanese Patent Laid-Open No. 2008-22198, the electronic shutter is used to perform shooting in a moving image capturing mode. When performing shooting in a still image capturing mode, the leading blade function of the mechanical shutter is implemented by the electronic shutter, and the trailing blade function is implemented by the mechanical shutter. The exposure times of the pixels arranged in the row and column directions are equalized by making the start of accumulation of charges in the pixels coincide with the traveling characteristic of the trailing blades. The pixels are almost simultaneously reset at a timing coinciding with the traveling characteristic of the trailing blades in the still image capturing mode, and the mechanical shutter almost simultaneously shields the light-receiving surfaces of the pixels.